It is known that certain microorganisms can selectively accumulate heavy metal ions from aqueous systems such as waste water effluents from metallurgical and mining operations. Thus, it is possible that these microorganisms may play an important role in the regulation of environmental pollution and in the recovery of useful metals from nature. The known processes which utilize these microorganisms are currently under intense scrutiny and investigation, but are, in general, not well understood.
Living cells have been known to concentrate cations from their aqueous environment. Further, microbial biomass has been demonstrated to exhibit a selective retention of heavy metals and transition metal elements as discussed above. It is believed that the ion exchange properties of the natural polysaccharides present in the walls of living cells may be at least partially responsible. Indications are that microbial cell walls possess the ability to bind with certain cations and inhibit them from becoming pollutants. An example of this type of process is taught by European Patent Application 0,094,979 (Skagerson) wherein there is disclosed a process for the retention of a high concentration of chromium in live yeast cells by intracellular adsorption. Another example of this type of process is taught by U.S. Pat. No. 4,320,093 (Volesky) wherein there is disclosed a technique for the removal of uranium and thorium cations from solution using a fungal microorganism of the genus Rhizopus.
It has been found, however, that the microorganisms used in these and similar metal recovery processes are destroyed by repeated contact with particulates and toxins from the waste water effluent to be treated. In particular, it has been found that fibrous biomass such as Penicillin sp disintegrates on repeated usage. The ability of these microorganisms to absorb and extract metals is reduced as the structure and integrity of the microorganisms are destroyed and, accordingly, they are not suitable for large scale operations where the microorganisms will be subjected to repeated contact with the dilute aqueous stream which typically comprise waste water effluent.
To enhance the commercial application of the microbial extraction of metals, attempts have been made to immobilize or stabilize the microorganism prior to exposure to the waste water effluent.
U.S. Pat. No. 4,021,368 (Nemec et al) discloses a process for retention of metal ions, such as uranium, from solution. More specifically, the process comprises using biomass of mycelia of fibrous fungi, stiffened by adding to the biomass polymerizible components, effecting polymerization thereof, and mechanically granulating the product.
A general review of procedures available for the immobilization of microorganisms is found in "Immobilized Cells and Organelles" by Mattiasson, B.; Vol. 1; CRC Press, Inc. More specifically, this reference addresses entrapment of cells with synthetic polymers such as polyacrylamide and polyurethane, and with natural polymers such as agarose and cellulose. It has been found, however, that polyacrylamide is a relatively difficult material with which to work. Complex equipment is required for reactions involving polyacrylamide and generally, the material is costly for immobilization techniques.